Conventionally, synthetic fibers such as polyethylene fibers and polyvinyl alcohol fibers are used as binder fibers for papermaking. Recently, papers made of polyester fibers in part or all as raw materials have been more commonly used because the polyester fibers have excellent physical properties such as mechanical property, electrical property, heat resistance, dimensional stability, and hydrophobicity, as well as cost advantage. Further, with expand in amounts employed and use application of the polyester fibers, there is a demand for binder fibers to have improved adhesiveness so as to make it possible to produce a paper with high strength.
Patent Document 1 (JP Laid-open Patent Publication No. 2013-174028) discloses an undrawn polyester binder fiber for papermaking. In order to obtain a paper with high strength, the undrawn polyester binder fiber has an intrinsic viscosity of 0.50 to 0.60, a single fiber fineness of 1.0 to 2.0 dtex, and a fiber length of 3 to 15 mm, wherein a salt of alkyl phosphate is applied to the undrawn fiber in a proportion of 0.002 to 0.05% by mass. Patent Document 1 describes that production of a fiber having a single fiber fineness of less than 1.0 dtex causes frequent fiber breakage due to small tenacity of monofilament, resulting in deterioration in water dispersibility of the obtained fibers.
Patent Document 2 (Japan Patent No. 3731788) discloses a spinning technology, wherein a molten polyester including a polymer such as a polymethyl methacrylate in a proportion of 0.1 to 5% by weight is discharged from a spinneret having 1000 holes or more, in order to avoid, between the inner and outer perimeters of the yarn, variations in physical properties such as orientation and crystallinity, as well as in dye affinities, and further to prevent an out-of-order situation in the process due to fiber breakage. According to Patent Document 2, this technology does not require complicated equipment modification.